I'm starting to feel really stupid here. I can't follow any of your "not"s - for example, WHY can't there be a 3 in row 5 column 1??? There's no 3 in column 1, no 3 in row 5 and no 3 in that 3X3 block. Likewise for the other ones you have said "not in" about. Is there some other logic I am totally missing?

OK now I'm seeing the additional logic steps I hadn't thought about before. Got this Xmas puzzle done anyway, and my first very hard one. Still have a lot to learn when it comes to the hidden pairs and triplets though - any rules of thumb on those?